You would need to do something like the following at the top of your
.htaccess file, before the existing front-controller (ie. before
# BEGIN WordPress).
RewriteRule ^([\w-]+)\.html$ /$1/ [R=302,L]
This assumes your "postname" consists of just the characters
RewriteRule pattern captures the postname before the
.html URL suffix and saves this in the
$1 backreference, which is used in the substitution string (2nd argument).
Note that this is a 302 (temporary) redirect. Only change it to 301 (permanent) when you are sure it's work OK, in order to avoid any potential caching issues. As always, clear your browser cache before testing.
You should use mod_rewrite (as opposed to mod_alias
RedirectMatch) in order to avoid conflicts with existing mod_rewrite directives. mod_rewrite executes before mod_alias, despite the apparent order of the directives in the config file.